Nanostructured dielectric composite materials

ABSTRACT

A nanocomposite material suitable for electrical insulation includes a polymer compounded with a substantially homogeneously distributed functionalized nanoparticle filler. The nanocomposite material is produced by compounding the polymer with the functionalized nanoparticle filler by imparting a shear force to a mixture of the polymer and filler capable of preventing agglomeration of the filler whereby the filler is substantially homogeneously distributed in the nanocomposite material. The electrical insulation may be adapted for AC or DC high voltage, and may also be adapted for low or medium voltage to prevent formation of water tree structures.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 11/044,489 filed onJan. 27, 2005, now U.S. Pat. No. 7,579,397, which is hereby incorporatedby reference.

BACKGROUND

The use of fillers in both thermoplastic and thermoset polymers has beencommon. The practice of filling polymers is motivated both by costreduction and by the need to obtain altered or enhanced properties (e.g.changes in thermal expansion coefficient, corona resistance, etc). Mostconventional filler materials have dimensions that are larger than 1 μm.Certain materials (e.g. inorganic oxides) are flow available innanometric dimensions.

Nanostructured dielectric materials have demonstrated advantages overmicron-filled polymer dielectrics. For example, an increase indielectric strength and a reduction in space charge have been documentedfor the case of nano-TiO₂ filled epoxy over micron size TiO₂ filledepoxy composites and titania filled polyethylene composites.Improvements in dielectric properties observed for nano-filled polymerscould be due to several factors: (i) the large surface area ofnanoparticles which creates a large ‘interaction zone’ or region ofaltered polymer behavior, (ii) changes in the polymer morphology due tothe surfaces of particles, (iii) a reduction in the internal fieldconcentration caused by the decrease in size of the particles, and (iv)changes in the space charge distribution and/or a scattering mechanism.

It is well known that polymer properties are altered near a surface. Thehigh surface area of nanoparticles, therefore, leads to a large volumefraction of polymer with properties different from the bulk (theinteraction zone). Depending upon the strength of the interactionbetween polymer and particle, this region can have either a higher orlower mobility than the bulk material. It has also been postulated thatthe free volume in such interaction zones differs from the bulk. Becausethese interaction zones are likely to overlap at relatively low volumefractions in nanocomposites, a small amount of nanofiller has been shownto result in both increases and decreases in glass transitiontemperature.

SUMMARY

The introduction of nanometric particulates into both thermoplastic andthermosetting resins has yielded materials with enhanced electricalproperties. Of particular interest are the enhanced dielectric breakdownand voltage endurance characteristics, and the mitigation of internalspace charges.

Nanocomposite material is provided that is adapted for electricalinsulation comprising a polymer having compounded therein asubstantially homogeneously distributed functionalized dielectricnanoparticle filler.

The nanocomposite material is suitable for the formulation of electricalinsulation, such as power cables, cable accessories, and the like, forhigh AC voltage, low AC voltage, medium AC voltage, and high DC voltage.

There is further provided a process for producing a nanocompositematerial adapted for electrical insulation comprising providing afunctionalized dielectric nanoparticle filler; drying the functionalizeddielectric nanoparticle filler; and compounding a polymer with the driedfunctionalized dielectric nanoparticle filler by imparting a shear forcecapable of preventing agglomeration of the nanoparticle filler, wherebythe nanoparticle filler is substantially homogeneously distributed inthe nanocomposite material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph (SEM) of a typical homogeneousdispersion of nanofiller in polymer

FIG. 2 is a scanning electron micrograph (SEM) of an inhomogeneousdispersion of agglomerated nanofiller in polymer.

FIG. 3 is a graph showing breakdown strength observed for purecross-linked polyethylene (XLPE) and nanofilled XLPE.

FIG. 4 a is a graph showing the change in permittivity as a function offrequency for unfilled XLPE, and micron filled and nanofilled XLPE.

FIG. 4 b is a graph showing the change in tan δ as a function offrequency for unfilled XLPE, and micron filled and nanofilled XLPE.

FIG. 5 is a graph showing a comparison of electrical breakdown strengthof XLPE-based nanocomposites having different surface functionalization.

FIG. 6 is a graph showing voltage endurance characteristics forpolyethylene-based nanocomposites.

DETAILED DESCRIPTION

Nanocomposites are provided that are suitable for the formulation ofhigh voltage electrical insulation, such as power cables and cableaccessories. However, the applications for these nanocomposites are notlimited to these products, as there is a wide range of insulationapplications and products of interest to the power industry. Forexample, the electrical insulation containing the nanocomposite materialmay be adapted not only for AC or DC high voltage, but may also beadapted for low or medium voltage to prevent formation of water treestructures.

While high voltage cables generally have water protection, low or mediumvoltage electrical insulation may not be shielded from water. Theelectrical insulation containing the nanocomposite material can preventthe formation of undesired structures (referred to in the industry aswater trees) in the insulation polymer that develops when water contactslow or medium voltage lines.

Low voltage is typically up to about 5 kV, medium voltage is typicallybetween about 5 and about 60 kV, and high voltage is typically 60 kV andhigher. As a further example, the nanocomposite materials can beextruded or formed into tape for insulating super-conducting cables.

The nature of the nanocomposite material, or nanophase infilled polymermaterial, can be chosen to endow the product with unique and tailoredproperties (e.g. thermal or electrical conductivity, corona resistance,matched thermal expansion coefficient, etc.) without necessarilyincurring the penalties commonly experienced in this process.

The nanocomposite material adapted for electrical insulation comprises apolymer having compounded therein a substantially homogeneouslydistributed functionalized nanoparticle filler. The introduction ofnanometric particulates into both thermoplastic and thermosetting resinshas yielded materials with enhanced electrical properties. Of particularinterest are the enhanced dielectric breakdown and voltage endurancecharacteristics, and the mitigation of internal space charges whichresult when the particle size is reduced to approximately that of thepolymer chain length.

The advantages in terms of breakdown strength, voltage endurance,reduced permittivity, and mitigation of space charge have all beendemonstrated for the new formulations. Furthermore, processingparameters such as viscosity (as a function of shear rate),crystallinity, and others have been realized for the new materials, asthese attributes apply to the industrial scale processing of thesematerials.

In certain embodiments, polyolefins and ethylene propylene rubber areuseful where there is an interest in utilizing these benefits for powercables and cable accessories. In certain embodiments, the nanophasemedium of choice is silicon dioxide since a material of low dielectricloss is appropriate for these applications.

Nanocomposite materials that have functionalized nanoparticles have beenshown to have significantly improved properties. In this regard,functionalizing chemical agents that have been shown to be effectiveinclude aminosilane, hexamethyldisilazane (HMDS), andvinyltriethoxysilane. In particular, detailed chemical analysis hasshown vinyltriethoxysilane to be particularly suited for use withpolyethylene since the vinyl group will bond to the polymer while thesilane couples to —OH groups at the particle interface.

With respect to the processing of these materials, substantial shearforces are needed in their compounding in order to alleviateagglomeration. Other parameters for processing conditions includeparticulate pre-processing, compounding temperatures, cross-linking,molding conditions and post processing.

In the present specification, “agglomerated” means that some individualparticles adhere to neighboring particles, primarily by electrostaticforces.

Characterization of these materials allows informed tailoring of thenanofilled polymer materials. These include FT infra-red absorption,electron paramagnetic resonance, X-ray analysis, differential scanningcalorimetry, Raman spectroscopy, thermally-stimulated currents,dielectric spectroscopy, and nuclear magnetic resonance. Engineeringthese materials is aided by establishing the correct conditions at theinterfacial zones.

“Nanoparticle” is defined as a particulate material having an averageparticle or grain size between 1 and 100 nanometers. Nanoparticles aredistinguishable from particles having a particle size in the micronrange, that is, greater than about 1 μm. Nanoparticles of any size, thatis, ranging from about 1 nm to less than about 100 nm, may be used inthe nanocomposites. In certain embodiments, particle size may range fromabout 2 nm to about 80 nm, optionally from about 5 nm to about 50 nm,and in other embodiments, from about 5 to about 30 nm.

Particle size distribution of the nanoparticles is typically narrow. Anarrow particle size distribution is defined as one in which greaterthan about 90% of the particles have a particle size in the range ofabout 0.2 to about 2 times the mean particle size. In certainembodiments, greater than 95% of the particles have a particle size inthis range, optionally greater than 99%. Another way to define aparticle size distribution is in terms of the mean particle size and thewidth of the distribution; this method is used in the nanoparticleindustry. The relationship between the width of the distribution curveat one half of the maximum value (full width-half max or FWHM) and meanparticle size is used as a measure of broadness or narrowness of thedistribution. For example, a distribution having a FWHM value that isgreater than the mean particle size is considered relatively broad.Specifically, a narrow particle size distribution is defined in terms ofFWHM as a distribution in which the FWHM of the distribution curve isequal to the difference between the mean particle size plus 40% of themean and the mean minus 40% of the mean. (This may be simplified to twotimes 40% of the mean, or 80% of the mean. Using this simplifiedformula, the FWHM is less than or equal to 80% of the mean.) In certainembodiments the FWHM is less than or equal to the difference between themean plus 30% and the mean minus 30% (60% of the mean.). In otherembodiments, the FWHM is less than or equal to the difference betweenthe mean plus 20% and the mean minus 20% (40% of the mean).

Nanoparticles useful in the nanocomposites may be equiaxed, such thattheir shape is quasi-spherical. The long axis of a particle is definedas the longest axis through a particle, and the short axis means theshortest axis through a particle. In certain embodiments, the long axisof the nanoparticles for use in the nanocomposites adapted forelectrical insulation is approximately equal to the short axis,resulting in a particle shape that is quasi-spherical. In theseembodiments, for at least about 90% of the nanoparticles, the ratio ofthe length of the short axis to that of the long axis is at least about0.1, optionally about 0.4, and further optionally about 0.8.

Non-spherical nanoparticles may also be used in the nanocompositesadapted for electrical insulation. In this case, particle size isdefined as the size of the smallest dimension of the particle. Forexample, nanotubes having an average particle diameter of about 1 toless than 100 nm may be used, and particle size of such particles is theparticle diameter, about 1 to less than 100 nm. Other non-sphericalnanoparticles that may be used in nanocomposites adapted for electricalinsulation include carbon or ceramic nano-fiber whiskers.

A nanocomposite material adapted for use in electrical insulationcomprises a polymer having compounded therein a substantiallyhomogeneously distributed, functionalized, dielectric nanoparticlefiller. The nanoparticle filler may be at least one of a metal boride, ametal carbide, a metal carbonate, a metal hydroxide, a metal nitride, ametal oxide, a mixed metal oxide, a metal silicate, a metal titanate, asilica, a carbon nanotube, or carbon or ceramic nano-fiber whiskers.

In certain embodiments, the nanoparticle filler is at least one ofalumina, aluminum hydroxide, aluminum nitride, barium oxide, bariumstrontium titanate, barium titanate, calcium borate, calcium carbonate,calcium oxide, glass fibers, glass particles, kaolin clay, magnesiumaluminum silicate, magnesium calcium carbonate, magnesium calciumsilicate, magnesium hydroxide, magnesium oxide, silica, silicon carbide,sodium borate, strontium oxide, strontium titanate, talc, titania, zincoxide, zirconia, zirconium silicate, or mixtures thereof. Representativesilicas include, without limitation, quartz and amorphous silica, suchas fumed silica or precipitated silica.

By being functionalized, it is meant that the surface of thenanoparticle filler has been treated to result in the presence of afunctional moiety, such as at least one of an organosilane, anorganotitanate or an organozirconate, prior to preparing thenanocomposite. In certain embodiments, the moiety comprises an organicgroup selected from alkyl, alkylamino, alkoxy, amino, carboxy and vinyl,or combinations thereof. The nanoparticle filler may be treated, byknown methods, with a compound, such as a coupling agent, such that thenanoparticle filler surface moiety is a reaction residue of the couplingagent, in certain exemplified embodiments, of at least one ofaminosilane, hexamethyldisilazane, or vinyltriethoxysilane.

The coupling agent may be applied from a solution or the gas phase tothe filler particles. The coupling agent acts as interface between thepolymer and the nanoparticle filler to form a chemical bridge betweenthe two. Representative examples include organotrialkoxysilanes,titanates, and zirconates. Silane coupling agents may comprise silanesof the formula Si_(n)H_(2n+2) and other monomeric silicon compoundshaving the ability to bond inorganic materials, such as nanoparticlefillers, to organic resins. The adhesion mechanism is due to two groupsin the silane structure, a hydrolyzable group, usually an alkoxy groupand an organofunctional group. The Si(OR₃) portion reacts with theinorganic reinforcement, while the organofunctional (vinyl-, amino-,etc.) group reacts with the resin. The coupling agent may be applied tothe inorganic materials as a pre-treatment. Titanate and zirconatecouplers are a family of alkoxy titanates and zirconates that typicallyhave one to three pendant organic functional groups. The titanatecouplers may also act as plasticizers to enable higher loadings and/orto achieve better flow.

Organosilane compounds useful for modifying the surface of nanoparticlesmay have the formula R_(n)SiR_((4-n)) and contain n hydrolyzable Rgroups, where n is 1-3, which may be alkoxy groups; R may be alkyl,alkylamino, alkoxy, amino, aryl, cyano, carboxy, hydroxy, epoxy,mercapto, or vinyl. In certain embodiments the organic groups aremethyl, decyl, octyl, aminopropyl, and/or acetoxy. For hydrophobicpolymers, such as polyethylene, for example, non-polar alkyl groupshaving at least ten carbon atoms, may yield improved properties. Forhydrophilic polymers, such as epoxy resins, R may preferably containpolar functional groups such as amino or epoxy groups. Examples ofhydrophobic silanes that may be used include n-decyltriethoxysilane,dodecyltriethoxysilane, hexadecyltrimethoxy silane, orn-octadecyltrimethoxysilane. Examples of silanes containingorganofunctional groups includen-(2-aminoethyl)-3-aminopropyltriethoxysilane,n-(2-aminoethyl)-3-aminopropyltrimethoxy silane,3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane,methacryloxypropyltrimethoxysilane, methacryloxymethyltriethoxysilane,acetoxyethyltrimethoxysilane, (3-acryl-oxypropyl)trimethoxysilane,5,6-epoxyhexyltriethoxysilane, (3-glycidoxypropyl)triethoxy silane,(3-glycidoxypropyl)trimethoxysilane, 3-mercaptopropyltrimethoxysilane,3-mercaptopropyltriethoxysilane, 2-cyanoethyltrimethoxysilane,vinyltrimethoxysilane, vinyltriethoxysilane, allyltriethoxysilane, andn-(3-acryloxy-2-hydroxypropyl)-3-aminopropyltriethoxysilane.

The titanate and zirconate couplers may include tetraalkoxy titanates[Ti(OR)₄] and tetraalkoxy zirconates [Zr(OR)₄], where R is alkyl, suchas methyl, ethyl, propyl, isopropyl, n-butyl, or t-butyl, and organictitanates [R_(n)TiR′_((4-n))] and organic zirconates[R_(n)ZrR′_((4-n))]. The most common alkoxy groups are methoxy, andethoxy. An exemplary titanate coupler is titanium methacrylatetriisopropoxide.

Further, the surface of a nanoparticle utilized in the nanocomposites istypically chemically clean, that is, uncontaminated by residues fromchemicals used in the synthetic process. Methods that producenanoparticles from a gas phase, such as a gas condensation process, suchas that described in U.S. Pat. Nos. 5,128,081 and 5,320,800, thecontents of which are incorporated herein by reference, typically yielda clean surface. Nanoparticles made by wet chemical methods are oftencontaminated by residues from chemicals used in the process; theseparticles may be subject to a post-production clean-up process to yielda chemically clean surface. For example, many processes for theproduction of titanium dioxide particles involve the oxidation of TiCl₄to TiO₂. The surface of particles produced by this process containsresidual chloride ions from the TiCl₄. These residues may be removed bychemical cleaning processes, if desired. Nanoparticles produced by a gascondensation process are not contaminated by process residues, becauseno solvents, reagents or intermediates are used. Therefore, zinc oxideor titanium dioxide nanoparticles for use in the nanocomposites may beprepared by a gas condensation process.

Alkyl is intended to include linear, branched, or cyclic hydrocarbonstructures and combinations thereof, including lower alkyl and higheralkyl. Representative alkyl groups are those of C₂₀ or below. Loweralkyl refers to alkyl groups of from 1 to 6 carbon atoms, and includesmethyl, ethyl, n-propyl, isopropyl, and n-, s- and t-butyl. Higher alkylrefers to alkyl groups having seven or more carbon atoms, such as 7-20carbon atoms, and includes n-, x- and t-heptyl, octyl, and dodecyl.

Alkoxy or alkoxyl refers to groups of from 1 to 8 carbon atoms of astraight, branched, cyclic configuration and combinations thereofattached to the parent structure through an oxygen. Examples includemethoxy, ethoxy, propoxy, isopropoxy, cyclopropyloxy, and cyclohexyloxy.Lower alkoxy refers to groups containing one to four carbons.

The nanoparticle filler suitable for electrical insulating applications(i.e., “insulating effective amount”) may comprise up to about 20% byvolume of the nanocomposite material, optionally up to about 10% byvolume of the nanocomposite material, and further optionally up to about5% by volume of the nanocomposite material, for metal oxide particles,this is typically about 5% by weight.

The polymer used in the nanocomposite material may comprise at least oneof a rubber, a thermoplastic polymer, a thermosetting polymer or athermoplastic elastomer. In certain embodiments, the polymer maycomprise a polyolefin rubber, a thermoplastic polyolefinelastomer/plastomer, a silicone rubber, a crystalline thermoplasticpolymer, or a semi-crystalline thermoplastic polymer. Examples of suchpolymers include, but are not limited to ethylene propylene dienemonomer (EPDM) rubber, ethylene propylene rubber (EPR), polyethylene, orco-polymers of ethylene with at least one C₃ to C₂₀ alpha-olefin oroptionally at least one C₃ to C₂₀ polyene. A variety of diluents andadditives which are well known to those skilled in the art may be mixedwith the polymer resins including water, oils, antioxidants, couplingagents, cross-linking agents, diluents, pigments and dispersants.

In specific embodiments of the nanocomposite material, the polymer maycomprise at least one of polyethylene, EPR or EPDM, and the nanoparticlefiller comprises silica having a surface moiety of a reaction residue ofat least one of aminosilane, hexamethyldisilazane, orvinyltriethoxysilane. For example, discussed below is extensive testingof the nanocomposite material in which the polymer comprisescross-linked polyethylene, and the nanoparticle filler comprises silicahaving a surface moiety of a reaction residue of vinyltriethoxysilane,as well as aminosilane, and hexamethyldisilazane.

Other polymers that may be used as the polymer include epoxy,polycarbonate, silicone, polyester, polyether, polyolefin, syntheticrubber, polyurethane, nylon, polyvinylaromatic, acrylic, polyamide,polyimide, phenolic, polyvinylhalide, polyphenylene oxide, andpolyketone resins, homopolymers and copolymers and blends thereof.Copolymers may include both random and block copolymers.

Polyolefin resins include polybutylene, polypropylene and polyethylene,such as low density polyethylene (LDPE), medium density polyethylene(MDPE), high density polyethylene (HDPE), and ethylene copolymers;polyvinylhalide resins include polyvinyl chloride polymers andcopolymers and polyvinylidene chloride polymers and copolymers, andfluoropolymers; polyvinylaromatic resins include polystyrene polymersand copolymers and poly α-methylstyrene polymers and copolymers;acrylate resins include polymers and copolymers of acrylate andmethacrylate esters; polyamide resins include nylon 6, nylon 11, andnylon 12, as well as polyamide copolymers and blends thereof; polyesterresins include polyalkylene terephthalates, such as polyethyleneterephthalate and polybutylene terephthalate, as well as polyestercopolymers; synthetic rubbers include styrene-butadiene andacrylonitrile-butadiene-styrene copolymers; and, polyketones includepolyetherketones and polyetheretherketones.

Processing Nanocomposites

The nanocomposite material suitable for electrical insulation may beproduced by providing the appropriate functionalized dielectricnanoparticle filler, drying the functionalized dielectric nanoparticlefiller, and thereafter compounding a polymer with the driedfunctionalized dielectric nanoparticle filler. Compounding is carriedout by imparting a shear force to the mixture of the polymer andnanoparticle filler that is capable of preventing agglomeration of thenanoparticle filler. The high shear mixing is conducted such that thenanoparticle filler is substantially homogeneously distributed in thenanocomposite material.

Providing silica nanoparticles in polyethylene as an example, but notfor reasons of limitation, a suitable processing method to make and usethe nanocomposite material for electrical insulation articles can besummarized by the following steps:

-   -   1. Drying of the nanoparticles.    -   2. Mixing the silica nanoparticles with low density polyethylene        (LDPE) pellets at a shear rate capable of preventing        agglomeration of the silica nanoparticles filler in the LDPE        melt to form a uniform dispersion.    -   3. Pelletizing the compounded nanocomposite material.    -   4. Contacting the nanocomposite pellets or mixing the        nanocomposite melt with a cross-linking agent (for example,        Dicumyl Peroxide or DCP).    -   5. Forming the insulation article such as through extrusion or        pressing.    -   6. Crosslinking the article to form nano-silica filled        cross-linked polyethylene (XLPE).    -   7. Post-cure (degassing) treatment of the insulation article.

Molecular water is attached to the surface of nano-particles throughhydrogen bonding, such as to hydroxyl groups on the oxide surfaces ofthe nano-particles, among other mechanisms. Drying may be carried out toremove this surface adsorbed water, as well as any fugitive chemicalspecies remaining as a result of the nanoparticle surface treatment.

The importance of use of dried nanoparticle filler is demonstrated bythe SEM micrographs of FIGS. 1 and 2. FIG. 1 shows a typical homogeneousdispersion of nanoparticle filler in polymer, which nanoparticle fillerwas dried before being introduced into the polymer. FIG. 2 shows aninhomogeneous dispersion of agglomerated nanoparticle filler in polymer,which nanoparticle filler was not dried before being introduced into thepolymer.

The dried, functionalized nanoparticle filler may be mixed with thepolymer in a continuous or batch process at a temperature above the melttemperature of the polymer. The nanoparticle filler may be added withthe pelletized polymer resin, and further additions may be made, such asin a batch process, after melt, to provide a higher loading of material.The mixer, or extruder used to carry out the dispersion process shouldbe able to generate sufficiently high stresses or shear to facilitatehomogeneous dispersion of the nanoparticles without exceeding thepolymer degradation temperature. Proper dispersion can be verified in aquality control step using field emission scanning electron microscopy.Melt viscosity of the nanoparticle filled polymer may also be measuredin a quality control step using a capillary rheometer in order toevaluate processability. Torque and temperature may be measured duringmixing to control the process parameters. Temperature must be highenough to melt the polymer, but must not lower viscosity to the pointthat adequate shear cannot be maintained.

Following mixing, the compounded nanocomposite material may bepelletized by known methods, followed by contacting the pellets with thecross-linking agent, such as by spraying, dipping, soaking and the like.The cross-linker coated nanocomposite pellets may then be melted at amild temperature and introduced into a heated vulcanization chamber ortube, according to known methods, such as with heating under nitrogen orsteam at an elevated pressure, to decompose the cross-linker and causecross-linking of the polymer. The insulation article, such as a cable ortape is then formed, and undergoes a cooling step, such as watercooling, also under pressure, to prevent byproducts from expanding voidsin the material. The insulation article may then be coiled on a reel,and degassed at mild heating conditions to evolve the gaseousbyproducts.

EXAMPLES

The base polyethylene used in the examples was a premium material fromBorealis (Rockport, N.J.) used in the manufacture of high-voltage (HV)extruded cross-linked underground cables. It is a high purity, filteredresin containing antioxidants.

The fumed silica used in the examples, of both micro- and nanometricsize, was sourced from Aerosil (Degussa Corp.) (Parsippany, N.J.).Surface modified nanosilica (aminosilane and hexamethyldisilazane(HMDS)) was also obtained from Aerosil. Some untreated nanosilica (AR200) was surface-modified with triethoxyvilylsilane in the vapor phaseby Polymer Valley Chemical, (Akron Ohio). FTIR characterization of theparticles suggests that the silane groups were covalently attached tothe nanoparticles. In addition, for the vinylsilane treatednanoparticles, the vinyl group reacted with the XLPE during processing,resulting in a covalent linkage between the XLPE and the vinylsilanetreated nanoparticles.

Dynamic vacuum drying of all the micro- and nano-particles was carriedout at 195° C. (except the vinylsilane-treated particulates which weredried at 160° C.) for 24 hours immediately prior to compounding. Thisdrying temperature was determined from thermogravimetric analysis (TGA).The composite was mixed with a Haake® melt mixer at and protocol toassure a uniform dispersion of the particles.

A sample with multiple recesses was used for breakdown strengthmeasurements and laminar samples were used for dielectric spectroscopy.All samples were created by hot pressing at a temperature of about 165°C. The pressure applied to the samples was initially 7 Mpa, which wasincreased to 25 Mpa after 25 minutes. After slow cooling under pressure,the samples were post-cured under vacuum at 70° C. for 72 hours to driveout the crosslinking by products, and were metallized with about 150 Åof sputtered gold.

The crystallinity and melting temperature of the processed samples weremeasured using Differential Scanning Calorimetry (DSC) at a heating rateof 10° C. per minute. Heating/cooling cycles were repeated twice foreach sample and the second peak was considered for calculation. Theweight of the sample for each experiment was approximately 5 mg. A setof four specimens was used for each type of sample, and the righttangential method was used to determine the crystallinity of thesamples. Table 1 summarizes the DSC tests.

TABLE 1 Degree of Crystallinity Melting Point Sample Type (%) (° C.)XLPE Only 44.6 ± 0.1 103.2 ± 1.4 5% Untreated 43.8 ± 1.0 109.0 ± 0.5nanosilica + XLPE 5% Aminosilane treated 45.8 ± 1.1 109.0 ± 0.6nanosilica + XLPE 5% HMDS treated 46.2 ± 1.4 109.0 ± 1.1 nanosilica +XLPE 5% Vinylsilane treated 60.1 ± 1.5 108.1 ± 1.1 nanosilica + XLPE

Breakdown strength measurements were conducted for nanocomposites andcompared to the base resin. A conventional Weibull distribution was usedto analyze the breakdown data for samples ranging in thickness from 0.15mm to 0.015 mm. The cumulative probability P of the electrical failureis represented by formula (1):

$\begin{matrix}{P = {1 - {\exp\left\lbrack {- \left( \frac{E}{E_{0}} \right)} \right\rbrack}^{\beta}}} & (1)\end{matrix}$where β is a shape parameter and E₀ is a scale parameter that representsthe breakdown strength at the cumulative failure probability of 63.2%.Breakdown tests were conducted at four different temperatures (25° C.,60° C., 70° C., and 80° C.) to study the effect of temperature on thebreakdown phenomenon.

Dielectric spectroscopic measurements were conducted for base polymer,micro-filled, nano-filled, and surface modified nano-filled compositesat various temperatures using a Novocontrol Alpha Analyser (type K) incombination with a Novocontrol active BDS-1200 sample cell. Laminarsamples, each approximately 0.5 mm thick with gold sputtered electrodeson both sides matching an electrode diameter of 2.2 centimeter were usedfor the measurements.

Results

One of the major factors controlling the properties of nanocomposites isdispersion of the nanofillers. FIG. 1 shows an SEM micrograph of atypical homogeneous dispersion observed in the nanocomposites tested. Incomparison, FIG. 2 shows an SEM micrograph of nanofiller heterogeneouslydispersed in polymer, revealing agglomerates of nanofiller in thepolymer. FIG. 3 shows the breakdown strength observed for the pure XLPEand the nanofilled XLPE. There is an increase in breakdown strength overthe base resin for all the nanofilled composites, but the largestincrease was observed for the vinylsilane treated silica/XLPE composite.In addition, the vinylsilane treated silica/XLPE samples maintainedtheir breakdown strength at elevated temperature, decreasing by a factorof 2 at 80° C., as shown in Table 2. For all other samples, the strengthdecreased by a factor of about three as the temperature was increased to80° C.

TABLE 2 Temperature Materials 25° C. 60° C. 70° C. 80° C. XLPE 269.30183.59 129.20 79.37 XLPE + Untreated 314.60 260.90 213.48 83.60nanosilica XLPE + Aminosilane- 400.10 266.20 263.17 134.55 treatednanosilica XLPE + HMDS- 336.60 225.20 208.00 128.60 treated nanosilicaXLPE + Vinylsilane- 446.63 422.23 344.18 220.60 treated nanosilica

FIGS. 4( a) and 4(b) show the change in permittivity (∈′) and tan δ as afunction of frequency for the unfilled XLPE as well as the micron filledand nanofilled XLPE. FIG. 4( a) shows that the high-frequencypermittivity of the untreated nanosilica composite is significantly lessthan the micron filled composite and somewhat less than the base resin.Surface treatment increases the permittivity, in theory due to thepolarity of the surface modifier. The tan δ behavior shows reduced lossfor some of the surface modified nanosilica composites and significantlyless loss for all the nanosilica composites at operating frequencycompared to the micron filled composite.

Although not intending to be bound by theory, the effect ofnanoparticles on breakdown strength and permittivity could be due to thesize of the particles which creates both a large surface area ofnanoparticles (large interfacial region) and a reduction in the internalfield concentration, changes in the polymer morphology, and changes inthe space charge distribution and/or a scattering mechanism. Thehypothesis is that the small size of the particles resulting in a lowerinternal field concentration coupled with the impact of the largesurface area dominates the changes in breakdown strength and dielectricresponse. This is supported by the crystallinity data. While it is wellknown that crystallinity affects the breakdown properties of XLPE, it isnot the dominant factor here.

Table 1 shows the degree of crystallinity in the samples prepared above.There is no significant difference in crystallinity except for thesample with silica nanofiller modified with vinylsilane, which resultedin a 50% increase in crystallinity. While the vinylsilane-treatednanoparticles did result in the composites with the highest breakdownstrength, the largest increase in breakdown strength was fornanoparticles over micron scale particles where no significant change incrystallinity occurred. However, there could be change in localcrystallinity due to adsorption of polymer chains to the particle, andfor nanoparticles, the probability of the same chain adsorbed onto theparticle more than once is low as the size of the particle is comparableto the chain confirmation length. This helps in reducing the internaldefects in nanocomposites, which acts as the precursor for thebreakdown. In vinylsilane-treated nanocomposites the chains arecovalently attached to the nanoparticles, in probability reducing theseinternal defects even further.

Several factors point to the dominance of the interface. First, thepermittivity of the nanoparticle/XLPE composites is lower than the baseresin. This implies that the interface is creating a reduction in freevolume that reduces the atomic and electronic polarization (as evidencedby the T_(g) changes in Table 1). This reduction in mobility might alsooccur in micron scale particle composites, but the volume of interfaceis too small to have an effect. In addition, there is significantinterfacial polarization in the micron particle composites resulting indispersion at about 70 Hz. This is mitigated in the nanoparticlecomposites, particularly for nanoparticles treated with aminosilane orHMDS.

The breakdown strength is also significantly impacted by the interface.An extensive electron paramagnetic resonance data taken on this systemsuggests that there is a higher concentration of oxygen radicals on thesurface of nanoparticles which increases further in the nanocomposites.This shows the ability of the nanoparticles to hold surface chargeswithout quantum tunneling. This high charge on the nanoparticle surfacecould create an envelope of positive counter ions outside it, making anelectrical double layer. Even at low volume fraction of particles, theseregions could overlap creating a local conductive pathway reducing thebuildup of space charge. Surface modification of nanoparticleseffectively increases the screening length by altering the chargedistribution and thereby improving the breakdown strength.

The vinylsilane-treated nanoparticles should lead to a higher crosslinkdensity in the nanocomposites. This would explain why the breakdowndrops less at the glass transition temperature, as the crosslinksinhibit polymer mobility. Further, the large interfacial area ofnanocomposites provides superior breakdown strength and permittivityover microcomposites.

FIG. 5 shows a comparison of electrical breakdown strength of XLPE-basednanocomposites with different surface functionalization measured atdifferent temperatures. The vinylsilane-treated silica nanoparticlesexhibited the highest breakdown strength. FIG. 6 shows voltage endurancecharacteristics for polyethylene-based composites as a function of time.Vinylsilane modified nanosilica particles at 5% loading providedenhanced voltage endurance characteristics.

The above results demonstrate the suitability for electrical insulationand the improvement in dielectric properties obtained using ananocomposite material comprising a polymer having compounded therein asubstantially homogeneously distributed functionalized dielectricnanoparticle filler.

While the present invention has been explained in relation to certainembodiments, it is to be understood that various modifications thereofwill become apparent to those skilled in the art upon reading thespecification. It should be understood that the embodiments describedabove may be practiced in the alternative, or in combination, asappropriate. Therefore, it is to be understood that the presentinvention is not limited to the specific embodiments described above,but includes variations, modifications defined by the following claimsand equivalent embodiments. All such modifications and variations areintended to be included within the scope of the invention as defined inthe appended claims.

1. A process for producing a nanocomposite material suitable for a powercable electrical insulation article comprising: providing afunctionalized dielectric nanoparticle filler wherein the nanoparticlefiller is at least one of silica, titania, kaolin clay, or magnesiumoxide; drying the functionalized dielectric nanoparticle filler;compounding a polymer with the dried functionalized dielectricnanoparticle filler by imparting a shear force capable of preventingagglomeration of the nanoparticle filler, whereby the nanoparticlefiller is substantially homogeneously distributed in the nanocompositematerial; pelletizing the compounded nanocomposite material; melting thepelletized nanocomposite material; and, forming the insulation article.2. The process of claim 1, further comprising contacting the pelletizednanocomposite material with a cross-linking agent, wherein thecross-linking agent coated pelletized material is capable of melting andforming the insulation article.
 3. The process of claim 1, furthercomprising mixing a cross-linking agent in a melt of the pelletizednanocomposite material.
 4. The process of claim 1, wherein the surfaceof the nanoparticle filler has an organosilane functional moiety;wherein the moiety optionally comprises at least one of alkyl,alkylamino, alkoxy, amino, aryl, cyano, carboxy, hydroxy, epoxy,mercapto, vinyl, or combinations thereof.
 5. The process of claim 4,wherein the nanoparticle filler surface moiety is a reaction residue ofat least one of aminosilane, hexamethyldisilazane,vinyltrimethoxysilane, or vinyltriethoxysilane.
 6. The process of claim1, wherein the polymer comprises at least one of polyethylene, ethylenepropylene rubber, ethylene propylene diene monomer rubber, a co-polymerof ethylene with at least one C₃ to C₂₀ alpha-olefin, or a co-polymer ofethylene with at least one C₃ to C₂₀ polyene.
 7. The process of claim 6,wherein the polymer comprises polyethylene.
 8. The process of claim 1,wherein the nanoparticle filler has a particle size ranging from 2 to 80nm.
 9. The process of claim 1, wherein the nanoparticle filler comprisesup to about 20% by volume of the nanocomposite material.
 10. The processof claim 1, wherein the nanoparticle filler comprises up to about 10% byvolume of the nanocomposite material.
 11. The process of claim 1,wherein the nanoparticle filler is silica, optionally, wherein thesilica comprises at least one of quartz, amorphous silica, fumed silica,or precipitated silica.
 12. A method to make a nanocomposite materialfor an electrical insulation article comprising: drying a functionalizeddielectric silica nanoparticle filler; compounding by mixing the silicananoparticle filler with low density polyethylene pellets at a shearrate capable of preventing agglomeration of the silica nanoparticlefiller to form a substantially homogeneous, uniform nanocompositematerial dispersion; pelletizing the compounded nanocomposite material;contacting the nanocomposite pellets with a cross-linking agent ormixing a cross-linking agent in a melt of the pelletized nanocompositematerial; forming the insulation article; and, cross-linking theinsulation article to form silica nanoparticle filled cross-linkedpolyethylene.
 13. The method of claim 12, wherein the functionalizeddielectric silica nanoparticle filler comprises vinylsilane treatedsilica nanoparticles, further comprising forming a covalent linkagebetween the cross-linked polyethylene and the vinylsilane treated silicananoparticles.
 14. The method of claim 13, wherein the vinylsilanetreated silica nanoparticles comprise at least 5% by volume of thenanocomposite material.